In an electric power steering system for a vehicle, as disclosed in JP 2002-249063A, a torque sensor is provided between an input shaft rotatable with a steering wheel and an output shaft such as a pinion shaft operatively coupled with tires. This torque sensor has a torsion bar, which connects the input shaft and the output shaft, and detects torque based on a twist angle of the torsion bar.
The system determines assist torque, which is to be generated by an electric motor, based on the torque detected by the torque sensor. A final assist torque is determined not only based on the detected torque but also various values of phase compensation, inertia compensation, damping control and return control. The phase compensation is for stabilizing a system by providing the detected torque signal with a phase lead. The inertia compensation is for compensating response delay caused by inertia of the motor and the like. The damping control is for improving convergence of the steering wheel. The return control is for improving steering operation at the time of returning the steering wheel.
The torque detected by the torque sensor and the above various compensations and controls do not reflect reverse transfer torque, which is transferred in reverse from road surface to the steering wheel through tires. As a result, steering operation feeling is not satisfactory.